Doc improvement: document interface omission in zmq_pgm.txt

The PGM transport supports the omission of the network interface to
select the default one like:

  announce.connect("epgm://eth0;239.255.128.46:64646"); // Use eth0
  announce.connect("epgm://239.255.128.46:64646"); // Use the default

Also, mention C++ in the additional community bindings of 0MQ in
zmq.txt.

doc/zmq.txt

@@ -208,8 +208,8 @@ by C programmers. The intent is that programmers using 0MQ from other languages
 shall refer to this documentation alongside any documentation provided by the
 vendor of their language binding.
 
-Language bindings (Python, PHP, Ruby, Java and more) are provided by members
-of the 0MQ community and pointers can be found on the 0MQ website.
+Language bindings ($$C++$$, Python, PHP, Ruby, Java and more) are provided by
+members of the 0MQ community and pointers can be found on the 0MQ website.
 
 
 AUTHORS

doc/zmq_pgm.txt

@@ -60,7 +60,8 @@ An 'interface' may be specified by either of the following:
 NOTE: Interface names are not standardised in any way and should be assumed to
 be arbitrary and platform dependent. On Win32 platforms no short interface
 names exist, thus only the primary IPv4 address may be used to specify an
-'interface'.
+'interface'. The 'interface' part can be omitted, in that case the default one
+will be selected.
 
 A 'multicast address' is specified by an IPv4 multicast address in it's numeric
 representation.